Post on 07-Nov-2014
Design of belt conveyor
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1. Introduction to material handling equipments
In any industrial process, the product being manufactured passes through
various phases and it needs to be transported from place to place. This could involve
processes such as transporting of raw material to the machines and then shifting the
machines from one station to another station and finally to the store or warehouse. This
involves the use of material handling equipment. Simplest form of material handling is to
take material from one place to another place manually or with the help of worker. In large
production setups, where the production rates are high and the product to be handled is
such that manual transportation is not possible, sophisticated material handling systems
would be required.
Material handling system does not contribute directly to the product value,
but it adds to the cost of the product and is therefore sometimes is referred to as a
necessary evil. In fact, least handling is the best handling.
1.1 Basic objectives
These basic objectives that a material handling system should fulfill are:
1. Quick and precise pick-up of loads.
2. Quick and efficient transfer of load with planned time interval.
3. Transport of loads in planned quantity.
4. Safe transport without any damage.
5. Accuracy in delivering at the destination.
6. Automation with minimum human element.
7. Low initial and operational costs.
8. Simple and easy to maintain.
9. Safe operation.
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1.2 classifications
The material handling system, based on design and operational
characteristics can be broadly classified in to three groups as shown below:
Material handling equipment
I. Hoisting equipment
a) Pure hoisting equipment: jacks, winches, pulley blocks etc.
b) Cranes: EOT cranes, jib cranes etc.
c) Elevators: lift elevator, bucket elevator etc.
II. Conveying equipment: Belt conveyors, Chain conveyors, Screw conveyors, Apron
conveyors.
III. Surface and overhead equipment: Fork lifts, Trucks, Railway cars, Overhead mono-
rails.
1.3 Basic principles of selecting material handling system
1. Direction of load travel.
2. Length of load travel.
3. Properties and characteristics of the material being handled.
4. The rate of flow of material.
5. Kind of the production process.
6. Method of loading and unloading.
7. Existing layout and conditions of the work space.
8. Initial and operational costs.
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1.4 Some important material handling system
1.4.1 Conveyor • Belt Conveyor
• Apron Feeder
• Screw Conveyor
• Deep Pan Conveyor
• Drag Chain Conveyor
• Flexowell Conveyor
• Rope way Trolley
• Skip Charging System
1.4.2 Stacker Reclaimer
• Linear stacker Reclaimer
• Bridge type reclaimer
• Circular stacker cum reclaimer
• Bucket wheel stacker cum reclaimer
1.4.3 Wagon tippler
• Side discharge
• Central discharge
1.4.4 Vibro Screen
• Linear movement
• Circular movement
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2. Belt conveyors
A belt conveyor consists of an endless belt of a resilient material connected
between two pulleys and moved by rotating one of the pulleys through a drive unit
gearbox, which is connected to an electric motor. The driving pulley end is called as head
end, and the pulley is called as head pulley. Conversely, the other pulley is at the tail end
and is referred to as the tail pulley as shown in figure 2.
Material is conveyed by placing it on the belt, through a feeder. As the belt
rotates, the material is carried with it on the other end, where it is then dropped in the
discharge chute. It should be noted that discharge can be arranged at any point along the
run by means of special discharge devices.
As the belt rotates, due to the weight of the belt and the conveyed material,
the belt will sag. To support this sag, rollers called as idlers or idler pulleys are placed on
both sides (carrying side and the return side). Closely spaced idlers are placed at the
loading point, as there is some impact due to the falling material and overcrowding of the
material in this region. The belt is subjected to tension and it being from a resilient material
is prone to elongation. This reduces the tension in the belts. Reduction in tension causes
slackness of the belt on the pulleys resulting in slippage and loss in power. To
compensate for this, a tensioning device called as take-up arrangement is used.
Figure 2 Belt conveyor
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2.1 Types of belt conveyors
I. Channel Stringer Belt Conveyors
II. Truss Frame Conveyors
III. Slider Bed Belt Conveyors
IV. U-Trough Belt Conveyors
V. Flat Slide Belt Conveyors
VI. Totally Enclosed Belt Conveyors
VII. Custom engineering conveyors
2.2 Advantages of belt conveyor over other system
1. Can be operated over long distances over any kind of terrain.
2. Having high load carrying capacity and carry all kinds of loads.
3. Noiseless as compared to chain conveyors.
4. Much simpler to maintain and don’t require any major lubrication system like chain
conveyors.
5. Their reliability has been proved over a long period by its use in the industry.
6. Environmentally more acceptable.
7. Low labor and low energy requirements.
8. Unlike screw conveyors, belt conveyors can be easily used for performing
processes functions in a production line.
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2.3 Types of conveyor layout
(A) Horizontal
(B) Inclined upwards
(C) Inclined upwards – Horizontal
(D) Horizontal- Inclined upwards
(E) Horizontal Inclined Horizontal
(F) Inclined Horizontal Inclined
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2.4 Methods of loading and discharging
Consideration of assumption:
1) The material should be placed centrally on the belt.
2) The material should be fed in the direction of belt travel and at a speed as near as
possible to that of the belt.
A) Hopper based loading.
B) Processing unit based loading.
C) Loading from a preceding conveyor
i) Head and discharge
ii) Both end discharge
iii) Plow discharge
D) Tripper discharge
2.5 Major equipments of belt conveyor
i. Conveyor Belt
ii. Pulleys
iii. Idlers
iv. Coupling
v. Bearing
vi. Drive unit
vii. Electric motor
viii. Cleaning device
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2.6 Requirement of belt which is to be used in belt conveyor
2.6.1 High strength: The belt is subjected to tensile loads. It is also subjected to other
loads due to scrapers, plows. The material fed also creates an impact load on the belt. All
these conditions require the belt to have high strength.
2.6.2 Low self weight: The belt is continuously driven on the pulleys. The power
requirement to drive this belt is dependent on its weight.
2.6.3 High wear resistance: The belts are subjected to rough working conditions over a
long period of time. Besides this, scrapers, plows, and other cleaners further create wear
as they rub over the belt surface. The belt should thus have a high wear resistance to
survive in tough conditions.
2.6.4 Low elastic and permanent elongation: Any elongation in the belt reduces the
tension created in the belt. This would reduce the power transmitting capacity of the belt
should have a low elastic and permanent elongation.
2.6.5 Flexibility: They should have a good flexibility in the longitudinal and lateral planes.
In many cases, belts are made to run over many pulleys. The belt material should have the
necessary flexibility to mould over the idlers.
2.6.6 High resistance to ply separation: Belts are made from plies, which are bonded
with a rubber element. The bonding of the plies should be such that it doesn’t separate out
due to the repeated bending of the belt over the pulleys.
2.6.7 Low water absorption capability: Water if it gets absorbed by the belt increases
the weight of the belt. This would result in increased power consumption and reduced
conveying capability. It also gives more dimensional stability of the belt.
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2.6.8 Suitable working environmental conditions: Humidity, extreme heat or cold. The
belt material should be good enough to ensure that it works with optimum results under
such working environmental conditions.
2.7 Introduction to Troughed Belt Conveyor:
There are many possible variations in the design of a troughed belt conveyor
depending on the purpose and duty for which the conveyor is being designed. Similarly the
choice of individual components, features and accessories found on a conveyor should be
selected on the basis of the functions which have to be performed by the conveyor.
Troughed belt conveyors offer an efficient means of transporting materials in large
quantities (bulk), over distances ranging from a few meters to several kilometers,
continuously.
As will be seen below, troughed belt conveyors are only one of the types of
belt conveyors available in the market today however, the troughed belt conveyor takes
numerous forms and is used in many different applications with tremendous success.
It is important to draw a distinction between bulk handling of materials and
unit handling. The former refers to the transportation of particulate product(s) on a
continuous basis for example, the conveying of lumpy ore from a mine to a processing
plant or for transporting coal from a stockyard to a bunker above a crusher.
'Unit handling' on the other hand is generally described as discontinuous as
this involves the transportation of for example, packed boxes, filled bags of cement and so
forth.
A troughed belt conveyor as described in this refers to conveyors which are
used to convey product in bulk.
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2.7.1 Types of Troughed Belt Conveyors
The term 'troughed' belt conveyor originates from the form of the carrying belt within the
supporting idler sets and differentiates this conveyor from alternative bulk handling belt
conveyor types which include 'Pipe', 'Sicon', 'Sandwich', 'Pocket or Sidewall', 'Cablebelt',
'Square', 'U-con' conveyors, etc.
Examples of these different types of conveyors can be seen below.
The type of conveyor to be used in any particular application depends on a number of
factors including the conveying route, properties of the material to be transported,
environmental considerations etc.
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3. Conveyor belt
Conveyor belt is made up of compounds comprised of natural rubbers,
styrene-butadiene rubber blends of natural and other synthetics, nitriles, butyl, ethylene
propylene-based polymer, polychloroprene, polybutadiene, polyvinyl chloride, urethanes
and silicones, etc., Each of those elastomers has specific usefulness for various ranges of
properties and operating conditions from which manufacturers and end-users can choose.
Conveyor belting and its corresponding cover composition can be designated
as either
(1) General Purpose Belting, or
(2) Special Purpose Belting.
Each of these two broadly classified groups should be further defined depending upon the
specific end use.
3.1 Constructional details Conveyor belts generally are composed of three main components:
1) Carcass
2) Skims
3) Covers
Nylon or EP Belt
Steel cord
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3.1.1 Carcass: The reinforcement usually found on the inside of a conveyor belt is referred
to as the carcass. The functions of a carcass include the following:
• Provide the tensile strength necessary to move the loaded belt.
• Absorbs the impact of the impinging material being loaded on to the conveyor belt.
• Provide the bulk and lateral stiffness required for the load support.
• Belts are connected at the ends by splicing them with belt fasteners. The carcass
should provide the necessary strength to hold fasteners.
The carcass is normally rated by the manufacturer in terms of maximum
permissible operating tension. The carcass can of two major types:
1. Fabric ply type
2. Steel cord type
3.1.2 Skims:
The rubber, PVC or urethane between the plies is called as skim. Skims are
important contributors to internal belt adhesions, impact resistance and play a significant
role in determining the belt load support and trough ability. Improper skims can give
reverse effect too. It can lead to ply separation failure.
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3.1.3 Covers:
They are used in conveyor belt construction to protect the conveyor belt
carcass and also to extend its service life. Its desirable properties such as:
1) Textures.
2) Clean ability.
3) A specific co-efficient of friction.
4) A specific color.
5) Cut resistance.
6) Enhanced impact resistance.
7) Hardness.
8) Fire, oil and chemical resistance.
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4. Conveyor pulley The most commonly used conveyor pulley is the standard steel pulley. They
are manufactured in a wide range of sizes and consist of a continuous rim and two end
discs fitted with compression type hubs. In most wide faced conveyors pulleys,
intermediated stiffening discs are welded inside the rim. Outer pulleys available are self
cleaning wing types, which are used at the tail, take up or snub locations where materials
tends to built up on the pulley face, and magnetic types which are used to remove tramp
iron from the material being conveyed.
4.1 Conveyor Pulley Assemblies
Conveyor pulley construction has progressed from fabricated wood, through
cast iron, to present welded steel fabrication. Increased use of belt conveyor has led
industry away from custom-made pulleys to the development of standard steel pulleys with
universally accepted size range, construction similarities, and substantially uniform load
carrying capacity for use with belts having a carcass composed of plies or layers of fabric.
“Standard” drum and wing pulleys are suitable for these applications. The present trend,
however, is to use higher tonnage conveyor system with wider, stronger belts that
incorporate a carcass of either steel cables or high strength tensile members. In these
applications, where high tensions are encountered, the use of custom made “engineered”
welded steel pulleys is dictated.
4.2 Type of pulley based on fabrication
1. Typical welded steel pulley.
2. Fabricated curve crown pulley.
3. Spun end curve crown pulley.
4. Lagged welded steel pulley.
5. Welded steel pulley with grooved lagging.
6. Slide-lagged pulley.
7. Lagged wing pulley.
8. Fabricated wing type pulley.
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4.3 Type of pulley based on Function
1) Driving pulleys (Head and Tail pulleys)
2) Snub pulleys
3) Idlers
a) Carrying idlers.
b) Return idlers.
4.3.1 Head pulley
Normally the discharge end of the conveyor where the material is
transferred to another conveyor is called as the Head end and the pulley in this end is
called the head pulley. Most of the cases the drive is attached to the Head end of the
pulley and so head pulley will designed stronger and bigger when compared to others.
Head pulley is rubber lagged to increase the grip or friction between belt and Pulley.
Figure: Head Pulley
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4.3.2 Tail pulley
The pulley which is situated in the receiving end of the conveyor is called as tail
pulley. Some times Screw take-up will be situated in this pulley. This pulley is movable
when take up is kept in this. When belt takes a turn for take-up arrangement or for any
other drive arrangement this term comes. This acts as a support when belt takes a turn.
4.3.3 Snub pulley
Snub pulleys are incorporated into the design of a conveyor in order to increase the
angle of wrap of the belt on the drive pulley. The greater wrap angle on the pulley allows
more power to be introduced into the belt as is passes around the drive pulley without slip
occurring. In this way, fewer drives are needed on longer conveyors or conveyors with
high conveying loads.
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5. Idlers
The needs for Idlers are to give proper support to conveyor and also to the Material
to conveyor. An endless conveyor belt in a conveyor structure is dragged from the tail
pulley where material is loaded onto the conveyor, to the head pulley or drive pulley where
the material is discharged. Between a conveyors' tail and head pulleys, whether the
distance is a number of kilometers or merely a few meters, the carrying and return strand
belting is supported on idler sets. The rolls are fitted with antifriction bearings with seals
and with adequate lubrication packed into it. The friction between the roller surface and the
belt makes the rollers to rotate and thus material is transferred from one point to another
through belt conveyor.
Figure 5 The arrangement of Idlers in a Belt Conveyor
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5.1 Idlers serve following functions
1) Support the belt and the conveyed material on the upper run and the belt in the
lower run with minimum frictional resistance.
2) Spacing of the idlers is reduced near the loading point, so as to support the belt
due to impact of material in that region. This would prevent the belt from wearing
quickly
3) Idlers help in centering the belt and guiding it to the drive and snub pulleys.
5.2 Type of Idlers
5.2.1 Carrying idler sets
These idler sets support the carrying-side (top) conveyor belt onto which the material is
loaded and transported. In the loaded zone we have Impact carrying Idler which is
covered by rubber material to absorb the loads as the loading or transferring points. Also
we have Self-aligning carrying idlers to avoid the belt off tracking.
Figure 5.2.1 carrying idlers
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Carrying or troughing idler sets usually comprise between two and five individual
idler rolls mounted into a common base, which is attached to the conveyor structure. Each
idler roll in a ‘set’ comprises its own set of bearings, seals, shaft and outer shell.
5.2.2 Return idler sets
These idler sets support the return-side (bottom) conveyor belt which returns to the tail
pulley after having discharged product over the head pulley.
The diagram shown above is flat return type of Idler where only one flat roller is
used. The return idler may also have more than one idler arrangement which is called as
Garland type idlers.
Figure 5.2.2 return idlers
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Once the material has been discharged from the carrying belt, the return belt is
guided back to the tail pulley on return idlers. The impact, carrying and return idlers are
spaced at different intervals. On the carrying-side, the mass of the belt plus the load
conveyed is greater than the mass to be supported on the return-side and thus, for the
tension in the conveyor belt (by the take-up and induced by the drive unit), the idler
spacing is selected accordingly. This 'sag' in the belt between the carrying and return idler
sets must therefore be designed on the basis of the heaviest load that the conveyor is to
transport.
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6. Coupling
6.1 Function
As the name indicates Couplings are the device used to couple or connect
two shafts, this is one of the most important component of any drive system. Since it is
impossible to maintain co linearity between two shafts couplings are designed to provide
better flexibility to allow initial or running shaft misalignment.
Following are the type of flexible couplings.
• Fluid Coupling
• Chain Coupling
• Geared Coupling
• Grid coupling
• Universal coupling
6.1.1 Fluid Coupling
There are three essential parts to a fluid coupling: the driving (input) section
known as the impeller the driven (output) section known as the runner and the casing
which bolts to the impeller enclosing the runner providing an oil tight reservoir. Both
impeller and runner each represent half of a hollow torus with flat radial vanes. At the inner
circumference a conical baffle is attached to the impeller and a flat baffle is attached to the
runner.
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These components comprise the working circuit. The operation of the fluid
coupling requires mechanical input energy, normally provided by a standard NEMA B
electric motor which is connected to the impeller and casing. The runner, which has no
mechanical connection with the impeller, is directly connected to the driven load. A variety
of mechanical connections; couplings, sheaves, and hollow shaft mountings, are available
to provide the mounting configuration best suited to the application. Finally the fluid
coupling must be initially charged by removing the fill (fusible) plug and adding the
recommended amount of oil based on the required torque.
Starting
Standard NEMA B motors are recommended when using fluid couplings and
will start virtually unloaded. Since the motor is mechanically connected to the impeller and
casing, the low inertia of these components and the oil are the only loads imposed. As the
electric motor accelerates to running speed, the impeller begins to centrifugally pump oil to
the stationary runner. Transmission of oil is diffused by the conical impeller baffle,
producing a gradual increase in torque, allowing the motor to accelerate rapidly to full
running speed. When all the oil is pumped into the working circuit, continuous circulation of
oil will occur between the impeller and runner forming a flow path like a helical spring
formed.
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As soon as the transmitted torque reaches the value of the resisting torque,
the runner starts rotating and accelerates the driven load. The time required to reach full
speed is dependent on the inertia of the driven load, the resistive torque, and the torque
being transmitted by the fluid coupling.
Running
The operation of a fluid coupling is based on the hydrokinetic principles and
requires that the output speed be less than the input. This difference in speed is called
slip. Further this principle provides that the output torque is equivalent to the input torque,
since windage and oil circulation losses are negligible. Therefore, efficiency equals 100%
minus the percent of slip. At full running speed fluid couplings will normally slip between
1% and 4%. The oil circulation between the impeller and runner has formed a vortex at the
outside circumference of the working circuit and is no longer deflected by the conical
baffle.
Overload – Stall
Should the load torque increase, the slip will increase, which causes the
runner to drop in speed. The vortex of oil circulating between the impeller and runner will
expand to provide additional torque. The extent to which this vortex can expand is limited
by the flat baffle on the runner. Consequently fluid couplings provide inherent overload
protection.
If the increase in torque causes the oil in the working circuit to expand to the point of
contacting the baffle, the coupling will stall and slip will be 100%. This continuous high slip
generates heat and the oil temperature will rise unless the overload is removed. When the
temperature rises to the temperature limit of the fusible plug, the core of the plug will melt,
release oil from the coupling and effectively disconnect power to the output shaft.
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To prevent the loss of oil the use of a proximity cutout switch or thermal trip
plug and limit switch is recommended. Coupling guards must be designed to permit air
circulation for cooling and to protect when oil is released from fusible plug due to overload.
6.1.2 Chain Couplings
Chain couplings operate similarly to gear couplings. Sprockets on each shaft
end are connected by a roller chain.
Figure 6.1.2 chain coupling
The clearance between its components as well as the clearance in mating the
chain to the sprockets compensate for the misalignment. Loading is similar to that of
geared couplings. Packed Grease Lubricants is primarily used with this types of
construction, necessitating a sealed sprocket cover. A detachable pin or master link allows
removal of the chain.
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6.1.3 Gear Coupling
Gear couplings compensate for misalignment via the clearance between gear teeth.
Figure 6.1.3 Gear Couplings
Shaft-mounted external gear teeth on both shafts mate with internal gear
teeth on a housing that contains a lubricant. Other designs mount external teeth on only
one shaft, mating with internal teeth mounted to the other shaft. Acceleration or
deceleration can result in impacts between gear teeth due to backlash from the clearance
being taken up on opposite sides of gear teeth. Misalignment will result in sliding relative
motion across mating teeth as they pass through each revolution.
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6.1.4 Grid couplings
External grid couplings use a corrugated steel grid that bends to compensate
for loading induced by misalignment.
Figure 6.1.4 Grid Coupling
Grooved discs attached to the ends of each shaft house the grid, which
transmits torque between them. Low amplitude sliding motion develops between the grid
and grooves as the grid deforms under load, widening in some locations and narrowing in
others over each revolution.
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6.1.5 Universal Coupling
Universal joints are used for maximum allowable misalignment up to 20 to 30
degrees, depending upon the specific design. They are used extensively for the drive
shafts of vehicles to allow the wheels to move with the suspension system. Universal joints
use a four-spindled component called the spider to connect two shafts terminating in
yokes or knuckles at right angles.
Figure 6.1.5 Universal Joint
Each of the four spider journals is supported by a bearing or bushing contained in one of
the knuckles, which allow articulation. In some cases, greater articulation can decrease
wear rates by allowing more complete development of a lubricating film.
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7. Bearing
As we all know, Bearings are used to give support the shaft of the roller or
idle pulley at both ends. They give also rotational motion of shaft by giving it support with
very less friction. Though some friction is taken place due to the metal to metal contact
inside the bearing between metal balls and metal casings, it is very negligible as compare
with the direct contact of rotating shaft and main frame of the conveyor.
Types of Bearings:
There are many types of bearings, each used for different purposes. These include
ball bearings, roller bearings, ball thrust bearings, roller thrust bearings and tapered roller
thrust bearings.
7.1 Ball Bearings
Ball bearings, as shown below, are probably the most common type of bearing. They are
found in everything from inline skates to hard drives. These bearings can handle both
radial and thrust loads, and are usually found in applications where the load is relatively
small.
Figure 7.1 Exploded view
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In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to
the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very
small point, which helps it spin very smoothly. But it also means that there is not very
much contact area holding that load, so if the bearing is overloaded, the balls can deform
or squish, ruining the bearing.
7.2 Roller Bearings
Roller bearings like the one illustrated below are used in applications like conveyer belt
rollers, where they must hold heavy radial loads. In these bearings, the roller is a cylinder,
so the contact between the inner and outer race is not a point but a line. This spreads the
load out over a larger area, allowing the bearing to handle much greater loads than a ball
bearing. However, this type of bearing is not designed to handle much thrust loading. A
variation of this type of bearing, called a needle bearing, uses cylinders with a very small
diameter. This allows the bearing to fit into tight places.
Figure 7.2 Cutaway view of a roller bearing
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7.3 Ball Thrust Bearing
Ball thrust bearings like the one shown below are mostly used for low-speed
applications and cannot handle much radial load. Barstools and Lazy Susan turntables
use this type of bearing.
8. Drive unit for belt conveyor
A) Direct gear motor drive
B) Drive through parallel shaft gear box
C) Drive through primary reduction by v belt and secondary by gear box
D) Drive through spiral bevel or worm gear box
9. Motor
Motor is a prime source of the energy to run the whole belt conveyor system.
By taking current, it produces the mechanical work and this mechanical work is given to
head pulley or tail pulley of the conveyor by means of gear box drive as discussed above.
We can also use an induction motor with variable speed drive by changing its frequency.
10. Cleaning device
An important property of the rubber covered conveyor belts is the high
coefficient of friction of rubber. This reduces the tendency of material to slip on inclines.
However it also increases the difficulty of cleaning the belt.
Some of the devices for belt cleaning are discussed below:
a) Belt scraper
b) Rotating belt cleaners
c) Water spray and wiper
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11. Problem
Design a belt conveyor to transfer 200 t/hour of foundry sand through a horizontal
distance of 20 meter. Foundry sand has density of 1.25-1.3 t/hour. Assume all the
related data for belt speed and angles.
Co-efficient of friction between belt drive roller and belt is 0.3.
What happens when the transfer of same material at some angle for the same condition?
Conclude from results.
Given data
Material is to be conveyed = foundry sand
Length of the conveyor = 20 m
Capacity of the conveyor = 200 tonnes
Type of the conveyor = horizontal
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Solution:
11.1 Design of belt
Important factors are to be considered:
a) Angle of repose and angle of surcharge b) Flow ability c) Effective belt width for material d) Volume capacity of belt, Q e) Mass capacity of belt f) Belt speed
11.1.1 Selection of belt width
• angle of repose of the material to be conveyed = 45 degree
• therefore surcharge angle = 30 degree
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11.1.2 Density of the material which is to be conveyed
• density = 1.25 – 1.3 tonnes/m3 (from following table)
Density and angle of repose of commonly conveyed materials
Material
Recommended max. angle of
belt to horizontal, deg
Density tones/m3
Angle of
repose,
deg
Anthracite, fine, dry 0.8-0.95 45
Gypsum, small lump 1.2-1.4 40
Clay, dry, lump 1.0-1.5 50
Gravel 12 1.5-1.9 45
Earth, dry 1.2 45
Foundry sand 24-26 1.25-1.3 45
Ash, dry 23 0.4-0.6 50
Lime stone, Lump 20 1.2-1.5 45
Coke 17 0.36-0.53 50
Wheat flour 23 0.45-0.66 55
Oat 18 0.4-0.5 35
Saw dust 27 0.16-0.32 39
Dry sand 18 1.4-1.65 45
Wheat 18 0.65-0.83 35
Iron ore 18-25 2.1-2.5 50
Peat 18 0.33-0.41 45
Coal(from mine) 18 0.65-0.78 50
Dry cement 20 1.0-1.3 50
Slag, anthracite 22 0.6-0.9 45
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11.1.3 Angle of throughing belt
Angle of throughing, β = 36 degree
The cross section of the lump on throughed belt of width ‘w’ is shown in figure
Area of c/s of lump,
A = 1 /2 ( .6 W + .6 W + 2 2 W c o s ) .2 W s in 1 / 2 ( .6 . .4 c o s )1 / 2 ( .6 .4 c o s )W W W W c o t
θ θ
θ θ φ
× ×
+ + +
Where,
= th ro u g h ed an g le = an g le o f rep o se
θ
φ
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Volume/meter length of the belt = A x l
This must be equal to volume of material to be conveyed/meter length of belt,
Mass/meter length of the belt L = ___________________________
density of the material A×
= mc/l
c/ = 1/2(.6w +.6w+2 .2w cos ) .2w sin
+1/2(.6w+.4w cos )1/2(.6w+.4w cos )cot
m ρ θ θ
θ θ φ
× ×∴------ 1
Where, mc = mass/meter length of belt
Ρ = density of material
If we know the values of mc, Ρ, θ and Ø then we can find width of the belt from standard
belt size data.
Finding belt size width,
Assume, travel speed initially = 2.5 meter/s,
Mass rate = 200 tonnes/hour
Ø = 36 degree
Θ = 45 degree
Design of belt conveyor
36
Therefore,
mc = mass rate to be conveyed / speed of travel ----------------------- 2
= (200 x 1000) / (3600 x 2.5)
= 22.22 kg
And the density of the foundry sand = 1.3 tonnes/m3 = 1300 kg/m3
Now putting this value in equation 1,
Therefore 22.22/1300 = 1/2 (.6w+.6w+2 x.2w cos36) x .2w sin36
+1/2(.6w+.4w cos36)1/2(.6w+.4w cos36) cot45
= (.7618w)(.1175w) + (.4618w)(.4618w)
= .3027w2
Therefore, 22.22 / 1300 = 0.3027w2
W min = 0.2376 m or 237.6 mm
This is minimum width required of belt, to avoid spillover select 400 mm width of belt which
is a standard one.
Now, let recalculate the equation 1, because according to belt width in mm belt speed may
vary.
So, for 400 mm belt width, maximum recommended speed, v = 2 m/s (from the table)
Design of belt conveyor
37
Now put this value in the equation 2,
mc = (200 x 1000) / (3600 x 2) = 27.8 kg
Now put this value of mc in equation 1,
27.8/1300 = 1/2 (.6w+.6w+2 x.2w cos36) x .2w sin36
+1/2(.6w+.4w cos36)1/2(.6w+.4w cos36) cot45
Therefore,
27.8/1300 = 0.3027w2
And so, w min = 0.265 m = 265 mm
Allowable belt conveyor speed in m/s
Belt width, mm Type of material conveyed
400 500 650 800 1000
Gravel, stone, coal, ash, ores 1.5 1.75 2 2.75 2.75
Coke, friable materials 1.25 1.5 1.5 1.75 2.0
Dry and wet sand, grains and
light materials
2 3 3.5 4 4
Abrasive materials, fine coke,
slag, crushed ore
1.25 2 2 2.5 2.5
Abrasive materials:
In large lumps, slaggy rock,
ores
- - 1.75 1.75 2
Design of belt conveyor
38
Here, we conclude that belt must require minimum 0.265 m width to avoid spill over select
400 mm belt width. 11.1.4 Mass of the belt/meter length of the belt mb = 5 kgf /meter (from table)
11.1.5 Length of belt for one passes,
Here conveyor is having horizontal layout and so the angle of elevation = 0 degree
Angle of elevation ά = tan -1 0/20 = 0
So, length of belt l = 20/cos 0 = 20 m
Weight of conveyor belts, kgf/meter
Belt width, mm 400 500 650 800 1000
Average weight
per meter run, kgf
5 6.5 9 12 16
Design of belt conveyor
39
11.1.6 Total length of belt passes,
(We can calculate this after the calculation of pulley diameter and the thickness of belt,)
We are getting,
Diameter of the pulley D = 500 mm =0.5 m
Thickness of the belt t = 8 mm = 0.008 m
So, Total length of the belt = D + 2 (t/2) + 20(2)
= 0.500 + 2(0.008/2) + 40
= 0.504 + 40
= 40.504 m
Design of belt conveyor
40
11.1.7 To check the no. of plies of belt,
Here, our belt width = 400 mm
And the density of conveyed material = 1300 kg/m3
So, from table
Table to check the number of plies of belt
Density of material conveyed, t/m3
over 0.4 0.85 1.25 1.75
Up to 0.85 1.25 1.75 2.5
Designation of belt fabric, oz
Belt
width,
mm
No. of
plies, i
28 32 28 32 36 42 32 36 42 48 32 36 42 48
Min 3 3 3 3 - - 4 - - - - - - - 400
Max 5 4 5 4 - - 4 - - - - - -
-
Min 3 3 4 4 3 - 4 4 - - - - - -
500
Max 6 5 6 5 5 - 5 5 - - - - - -
Min 4 4 5 4 4 3 5 5 4 - 5 5 4 - 650
Max 7 6 7 6 5 5 6 5 5 - 6 5 5 -
Min 4 4 5 5 4 4 5 5 4 3 6 6 4 4 800
Max 8 7 8 7 6 6 7 6 6 5 7 6 6 5
Design of belt conveyor
41
Min 4 4 6 5 5 5 7 6 5 4 8 7 6 5
1000
Max 10 9 10 9 8 8 9 8 8 7 9 8 8 7
Designation of belt fabric, oz 32
No. of plies i min = 4 i max = 4
Design of belt conveyor
42
11.1.8 Working tension in conveyor belt,
For oz 32, belt & mechanical joint and gravity take up joint belt,
Working tension per mm width of ply is taken from the table
Working tension in conveyor belts
Designation
28 oz
32 oz
36 oz 42 oz
48 oz
Vulcanized
joint &
gravity take
up
0.54
0.62
0.71
0.82
1.07
Working
tension,
kgf/mm
width per
ply
Mechanical
joint and
gravity take
up
0.48
0.57
0.66
0.77
0.98
Working tension is 0.57 kgf/mm width per ply.
Design of belt conveyor
43
11.2 Design of end pulleys
11.2.1 Diameter of pulley
The pulley dimension can be decided on the basis of the number of plies in the belt, the
minimum pulley diameter ‘D min’ can be roughly approximated by this formula:
D min = k x i where, k = multiplying factor which depends on the number of plies
= 125 (for 2 to 6 plies)
= 150 (for 6 to 12 plies)
i = number of plies
So, our calculation k = 125, because belt plies in our case are 4,
i = 4.
Now put these values in the main formula:
D min = 4 x 125 = 500 mm. 11.2.2 Width of the pulley
The pulley width can be taken as ‘W’
Formula: W = b + 2 x s where, b = belt width in mm
S = side margin (taken as 60 to 75 mm)
Assume, side margin s = 65 mm &
Calculated belt width b = 400 mm
So, W = 400 + 2 x 65
= 400 +130
W = 530 mm.
Design of belt conveyor
44
The calculated value should then be rounded off to the nearest standard size.
Some of the standard sized pulleys are listed in table, most of the preferred sizes are
based on the R 10 series of preferred numbers.
So, pulley width = 600 mm.
belt type/width/pulley diameter(mm)/piy 3 4 5 6 7 8 9 10 11 12
CC belt BE/BR 400 500 600 800 1000 1250 1250 1400 1600 1600NN100
250 315 400 500 630 800 1000 1250
NN150 250 315 400 500 630 800 1000 1250 NN200 315 400 500 630 800 1000 1250 1400 NN250 400 500 630 800 1000 1250 1250 1400 NN300 500 630 800 1000 1250 1400 1400 1600
NN belt
NN400 630 800 1000 1250 1400 1600 1600 1800 EP100
250 315 400 500 630 800 800 1000
EP150 250 315 400 500 630 800 800 1000 EP200 500 630 800 1000 1250 1400 1400 1600 EP250 630 800 1000 1250 1400 1600 1600 1800 EP300 630 800 1000 1250 1400 1600 1600 1800
EP belt
EP400 800 1000 1250 1400 1600 1800 We are selecting here NN type of belt for 500 mm diameter pulley and width of pulley is
600 mm.
So, belt type is NN 200.
Rubber cover property Code Tensile strength(Mpa) Elongation rate(%) Abrasion(mm3)
Scratch� split H ≥24 ≥450 ≤120 Abrasion D ≥18 ≥400 ≤100 Common L ≥15 ≥350 ≤200
Design of belt conveyor
45
11.3 Design of idlers: 11.3.1 Weight of revolving parts of idlers in kgf/ idler assembly, Our belt width = 400 mm so, assume that tube diameter of idlers = 125 mm and diameter
of antifriction bearing = 25 mm.
From table,
Weight of revolving parts of idlers in kgf/idler assembly
Tube diameter, mm
100
125
140
150
Belt
width,
mm
Bearing
diameter(antifriction),
mm
20 25
20
25
20
25
20
25
Troughing idles
400
500
13.6 14.1 16.4 20 - - - -
650 - - 17.9 21.6 24.4 24.6 - -
800 - - 19.4 23.1 26.2 27.4 - 34.7
1000 - - - - 29.9 30.2 - 37.8
Straight idlers
400
500
7.6 7.8 16.4 20 - - - -
650 - - 11 12.2 15.8 15.9 - -
800 - - 12.65 13.7 18.2 18.8 - 22.5
1000 - - - - 21.3 21.4 - 25.1
Then, the weight of throughing idler = 20 kgf/idler assembly, and The weight of the straight idler = 10.7 kgf/idler assembly
Design of belt conveyor
46
11.3.2 Maximum spacing for idlers in meter,
Belt width = 400 mm and
Density of conveyed material = 1300 kg/m3
From table,
Maximum spacing between idlers, m
Density of material conveyed,tonne/m3
Over 0.4 1.2 2.0
Belt
width,
mm
Up to 1.2 2 2.8
Return
idler
400 1.6 1.5 1.4 3.0
500 1.6 1.5 1.4 3.0
650 1.6 1.5 1.4 3.0
800 1.5 1.3 1.2 3.0
1000 1.5 1.3 1.2 3.0
So, max. Spacing for carrying idlers = 1.5 m And for return idlers max. Spacing = 3 m
Design of belt conveyor
47
11.3.3 For finding no. of both idler
a. carrying
b. return
Here, we have found max. Spacing between two idler assemblies for both carrying and
return idlers are a.5 m and 3 m respectively.
Our total length of conveyor is equal to 20 meters.
So, we can find out the total no. of carrying idlers and return idlers.
For carrying idlers = 20/1.5 = 13.33 =14 idlers assemblies.
For return idlers = 20/3 = 6.66 = 6 idlers assemblies.
Design of belt conveyor
48
11.4 Selecting the drive (head or both):
In this case, we have to select that drive which consists maximum intensity of
pressure. Higher the intensity of pressure more friction between the belt and the pulley can
be achieved. Because of that, better gripping of belt with the drive pulley can be achieved
and negligible chance of slipping of belt. Better power transmission by belt can be
achieved.
Pushing is better phenomenon than the pulling one and by calculations we
could find that the intensity of pressure per square mm of belt is higher on head pulley so
that we select head drive.
Design of belt conveyor
49
11.5 Estimation of power:
Work is required to raise the material against the gravity and to overcome the
rolling resistance between idler rollers and belt. It is very easy to estimate the work to raise
the material against the gravity. To estimate the rolling resistance, we have to calculate the
normal reaction between rollers of top run and bottom run. Normal reaction between top
run and idlers depends upon total mass of the material and belt on top run. While, for
bottom run it will depend upon mass of belt on roller. The co-efficient of rolling resistance
kr for such application lies between 0.15 and 0.30.
Normal reaction for top run Rt = g x m1x l x cos ά
Normal reaction for bottom run Rb = g x m2 x l x cos ά
Where m1 = mb + mc
m2 = mb
mb = mass of belt/unit length of belt
mc = mass of material conveyed/unit length of belt
l = length of belt
11.5.1 Mass of material conveyed /unit length of belt mc = mass rate to be conveyed /
speed of belt travel
= 200 x 1000 / (3600 x 2.0)
mc = 27.8 kg / meter and mb = 5 kg / meter
Therefore,
m1 = 27.8 + 5 = 32.8 kg / meter
Design of belt conveyor
50
Resistance to motion of top run Ft = Kr x Rt
= Kr x g x m1x l x cos ά
= 0.2 x 9.81 x 32.8 x 20 x cos 0
= 1287.07 N
Resistance to motion of bottom run Fb = Kr x Rb
= Kr x g x m2 x l x cos ά
= 0.2 x 9.81 x 5 x 20 x cos 0
= 196.2 N
Force to raise the material Fr = g x mc x l x sin ά
= 9.81 x 27.8 x 20 x sin 0
= 0 N
Therefore,
Power required = total force x speed
= (Ft + Fb + Fr) x v
= (1287.07+196.2+0) x 2
= 2996.54 W
Power of motor = power required / transmission efficiency
= 2996.54 / .75
= 3955.386 W
= 3.955 KW
Power of motor = 4 KW
Design of belt conveyor
51
Case 2: when the transfer of the given material is at 20 degree from the
horizontal surface,
Resistance to motion of top run Ft = Kr x Rt
= Kr x g x m1x l x cos ά
= 0.2 x 9.81 x 32.8 x 20 x cos 20
= 1209.45 N
Resistance to motion of bottom run Fb = Kr x Rb
= Kr x g x m2 x l x cos ά
= 0.2 x 9.81 x 5 x 20 x cos 20
= 184.36 N
Force to raise the material Fr = g x mc x l x sin ά
= 9.81 x 27.8 x 20 x sin 20
= 1865.50 N
Therefore,
Power required = total force x speed
= (Ft + Fb + Fr) x v
= (1209.45+184.36+1865.50) x 2 = 6518.62 W
Power of motor = power required / transmission efficiency
= 6518.62 / .75
= 8691.49 W
= 8.61 KW
Power of motor = 9 KW
Design of belt conveyor
52
12. Problem Conclusion
From this example of designing belt conveyor, we can conclude that at
transportation of material at some angle through belt conveyor system, consumes more
power than the transportation with zero angle to the horizontal plane.
Having some angle of inclination, increases the gravitational force with each
instance and thus the force to transfer the material from lower side to upper side is quite
power consuming.
Design of belt conveyor
53
13. Software for analysis
13.1 Introduction of pro-belt
Pro-belt computer software has been proven in the field to be highly accurate
for the design of all belt conveyors of any size or configuration. We can our pro-belt
calculations for a very long overland conveyor with a length of 12,345 meters. The
conveyor had a very small decline of 41 meters. This was an excellent example to use to
certify the accuracy of pro-belt because most of the belt tension and power requirement
came from friction calculations and very little came from lift requirements. High incline
conveyors are not a good example to use because lift power is a scientific absolute. The
calculations were made with our metric version, but the English version gives identical
results after units conversion.
The owner field tested this conveyor in 1995. The testing was extensive and
was considered to be highly accurate. Pro-belt gave a "Recommended Minimum Motor
Power" of 1746 kW versus the field test result of 1720 kW. The pro-belt analysis was
101.5% of the actual power requirement. This accuracy within 1.5% is considered to be
outstanding because this very long overland conveyor did not have much lift. The most
important calculation in belt conveyor design is to estimate the idler friction and belt flexure
friction accurately, since the lift calculation is a scientific absolute.
Pro-belt goes beyond CEMA to provide extremely accuracy results. It allows
belt conveyors to be divided into multiple sections with different loading conditions in each
section. The software gives identical accuracy whether the conveyor is divided into a few
sections or many sections. The designs are accurate for conveyors of any length including
extremely long overland belt conveyors.
Design of belt conveyor
54
The pro-belt pulley shaft design program was certified accurate by one of the
largest independent engineering firms in the world. The program calculates the deflection
at the pulley hub based on a free shaft analysis. The bending moment between the shaft
and hub is neglected. This gives a conservative design that eliminated many of the
common pulley failures. The program also performs a combined bending and torsional
fatigue analysis throughout the shaft in accordance with ANSI / ASME Standard B106.M.
The drive pulley extension is also analyzed for either directly connected drives or shaft
mounted drives. A warning message is displayed when the deflection or stress is outside
of allowable limits.
The pro-belt feeder program was tested by a world renowned bulk materials
handling equipment manufacturing company. They tested a very large belt feeder at
Kennecott Copper in Magna, Utah. The results were highly accurate and slightly on the
conservative side. They proceeded to use the pro-belt feeder program to design fifteen
(15) very large belt feeders for a large ore processing project in South America. Our feeder
program formulas are proprietary. Highly accurate results are produced for belt feeders of
any size operating with any bulk material. We will not publish the formula used to obtaining
this high degree of accuracy. The feeder program can also be used to model apron
feeders and drag chain conveyors. Belt feeders can be designed with either slider bed or
idler supported belts. The feeder and have a conveyor section as well as a feed hopper
section. The three page report gives the detailed results for all friction loads within the
feeder.
13.2 working
Pro-belt can be used to design horizontal curves in any troughing belt
conveyor of any length and belt sizes in either English or Metric units. The troughing belt
system and the return belt system are both included in the design. The troughing idlers
must be the 3-roller design of equal troughing angle on each lateral roll. The lateral rolls
can be longer than the middle roll to allow more drift, if desired. The idler troughing angle
is generally between 30 and 45 degrees. The return idlers must be a 2-roller "V" design.
Design of belt conveyor
55
Return idlers generally have a troughing angle between 10 and 30 degrees but more
commonly in the 15 to 20 degree range. Flat idlers cannot be used in horizontal curve
conveyors neither on the troughing belt nor on the return belt.
Pro-belt calculates the drift to the inner curve and drift to the outer curve
under any belt tension or loading condition. Maximum drift to the inner curve occurs at
maximum belt tension when the belt in the curve area is empty. The conveyor requires a
load case analysis with the conveyor loaded to the curve but empty thereafter. The
acceleration belt tensions should be used to check the curve design for maximum drift to
the inner circle. Tail drive regenerative conveyors would normally have a maximum inner
circle drift with the opposite conditions; i.e., stopping with the belt empty in the curve and
full thereafter. This program will design the curve for both loaded and empty conditions on
the same report. The return belt can also be checked for drift to the inner circle and the
outer circle.
Maximum drift to the outer curve occurs at minimum belt tension when the
belt in the curve area is loaded. The conveyor requires a load case analysis with the
conveyor loaded in the curve or throughout. The stopping belt tensions should be used to
check the curve design for maximum drift to the outer curve. Tail drive regenerative
conveyors would normally have maximum outer curve drift with the opposite conditions;
i.e., accelerating with the belt full throughout. Many other load case possibilities should be
considered.
Pro-belt allows faultless belt tracking of the conveyor belt by setting idler banking, tilt and
tracking.
13.3 nomenclatures
The Idler banking angle or super-elevation is the primary conveyor design
parameter used to provide a faultless tracking of the conveyor belt. The idlers are raised
on the inner side of the curve to provide a slope to the idler base. Banking angles of 8
degrees are common. A high banking angle will succeed in reducing the empty belt drift to
the inner curve but will also cause an increase in drift of the full belt to the outer curve.
Design of belt conveyor
56
Banking with an idler tilt up to 2 degrees has been a successful approach to curve design.
Provisions in the design should allow an adjustment of the idler banking, tilt and tracking in
the field, if necessary.
The Idler tilt angle is the secondary conveyor design parameter used to
provide a faultless tracking of the conveyor belt. The idlers are tilted forward at the top in
the direction of belt travel to provide up to 3 degrees of tilt to the idler base. The belt
tracking will probably not be as effective as indicated by the program when set above 3
degrees. A tilt angle of 2 degrees has been shown to be a successful approach to curve
design.
The Idler tracking angle is a tertiary conveyor design parameter used to
provide a faultless tracking of the conveyor belt. The idlers are moved forward in the
direction of belt travel on the inner side of the curve to provide up to 2 degrees of tracking
to the idler base. Tracking utilizes belt friction on the idlers to effect a movement in the belt
and is not the most reliable method for belt guidance. However, tracking can be used in
the field to make minor adjustments in the belt drift. A small tracking angle can make a
large change in the drift. The sum of tracking plus tilt should not exceed 3 degrees.
The Throughout belt factors are used to analyzes the effect in belt drift as a
result of belt trough ability. A belt which is stiff transversely has a reduced trough ability
and applies a greater force on the lateral idler rolls and a lower force on the central idler
roll. A belt with greater stiffness and lower trough ability actually improves belt tracking.
The Material follow factors represent the ability of the material to follow the
drift of the belt without moving toward the lower edge. A value of 1.0 represents a material
that has no movement when the belt drifts on the idlers. The value will be zero for very free
flowing materials that flow like water.
The Belt friction adjustment uses a friction multiplier to adjust the friction
between the belt and the idlers. This friction is very important in layouts which include tilt
and/or tracking of the idlers. The belt will not track as intended when the actual friction is
Design of belt conveyor
57
less that assumed. The friction multiplier can be changed to see the sensitivity of the final
horizontal curve design to friction.
The Allowable belt friction is calculated based on the input parameters.
The allowable belt drift is dependent on the unused space on the lateral idlers, belt edge
distance with the given material loading and material follow factors. The belt is allowed to
drift slightly beyond the edge of the lateral rollers by an amount of 1 inch or 25 mm. The
actual drift results is compared to the allowable belt drift and an "OK" or "FAIL" indicator is
given for each section point on the curve. These results are given for all three conditions:
1) troughing belt empty, 2) troughing belt loaded and 3) return belt.
13.3 advantages of analysis software (pro-belt)
a) It is used worldwide.
b) It is easy to use.
c) Will design any belt conveyor.
d) Allows any length without limit.
e) It is flexible.
f) Designs belt feeders, apron feeders & chain drag conveyors.
g) Designs pulley shafts.
h) Provides unlimited technical assistance.
i) It is very easily justified.
j) It is DOS based but easier and faster than Windows programs.
k) It is unquestioned in accuracy.
Design of belt conveyor
58
14. Dynamic analysis
For simple conventional belt conveying systems there are many well
documented procedures to calculate required powers, tensions and other factors. With the
assistance of computers, this rigid body analysis has been successfully streamlined,
allowing the designer to concentrate on the problems of chute design, drive house layouts
and components of manufacture.
However, with the growing need for larger, longer conveying systems, there is a need to
refine the analysis procedures. Generally, when conveying systems are analysed as rigid
bodies in a stationary state, the effects of boundary conditions - starting and stopping - are
ignored.
Dynamic Analysis is a computer simulation of the properties and
performance of the system in motion - an analysis of the starting and stopping
characteristics of a belt conveyor.
Explaining dynamic analysis
During steady state running..
Figure 1 shows a simple conveyor system during steady state running,
modeled as a series of masses and springs. In a steady state condition, the spring
extension before the drive (L1) and after the drive (L2) remains constant. In the steady
state condition, the torque due to the effective belt tension on the pulley is matched by the
torque produced by the drive.
Design of belt conveyor
59
FIG. 1 Mass-spring representation of
steady state running
The tension in the springs between the masses is determined by their stiffness and
extension. Since during steady state running, the distance between the masses (and the
spring extension) remains constant, the tension in the springs also remains constant.
After a coasting stop...
Figure 2 shows the same system, after the drive has been turned off.
1. The removal of the drive torque from the pulley leaves the torque unbalanced. This
results in a rapid deceleration of the drive pulley from v to (v._v), so that the rim of
the pulley moves a shorter distance than do the masses adjacent to it.
FIG. 2 Mass-spring representation of Coasting stops
Design of belt conveyor
60
2. This results in the shortening of the spring upstream of the pulley (with a resulting
decrease in tension) and a lengthening of the spring downstream and an increasing
in tension.
3. The change in tension on only one side of the masses adjacent to the pulley,
subsequently produces a force imbalance on these masses which causes them to
decelerate.
4. The deceleration causes changes in the extension and hence tension of the springs
on the other side of the masses. The resulting force imbalance causes the
disturbance to propagate further along the conveyor.
The resulting wave of decreased tension propagates down the carry side of
the conveyor and a wave of increased tensions propagates down the return side. For
simplicity, the variations in tension will be referred to as "compression" and "tension"
waves. These labels are not entirely accurate, since it is not possible to get true
compression on a conveyor belt, but the terms are widely accepted.
If the magnitude of the "compression" wave is greater than the actual steady-
state tension of a region of the conveyor through which the wave passes, highly non-linear
behavior will result. The belt tension in the region will not become negative but extremely
low tensions and large belt sag between idlers will occur. Destructive dynamic effects
frequently result from this type of occurrence.
Design of belt conveyor
61
FIG. 3 Mass-spring representation of coasting stop
with gravity take-up
Coasting stop with gravity take up
Figure 3 is the same as Figure 2, except that a take-up is located immediately
downstream of the drive. The increase in tension downstream of the pulley, instead of
inducing a tension wave, produces a force imbalance on the take-up, causing it to
accelerate upwards. This upward movement of the take up, absorbs the "tension" wave.
As a result, the return side of the belt is unaffected by the drive stopping until the
"compression" wave from the carry side has traveled completely around the conveyor.
The speed with which the initial waves propagate is a function of the system
mass and the belt axial stiffness. The loaded side of the belt will be heavier and
consequently waves on the carry side will propagate more slowly than waves on the return
side. Other important factors are the drive inertia and the belt stiffness. The steady state
velocity of the belt does not influence the magnitude of the stress wave.
Design of belt conveyor
62
14.2 When we can use it?
There are no cut off points to dynamically analyzing a conveying system. At
present it is usual to analyze large systems with complex profiles. Dynamic analysis tends
to be introduced for conveyors in excess of 1000m with capacities about 100 tph. This may
be the norm, however any conveying system experiencing large take-up movements or
adverse shock wave propagation resulting in premature pulley or drive failure, should fall
into the spectrum.
It is often possible to identify dynamic problems in a conveyor when large movements of
the take-up occur. These movements are related either to elastic stretch in the belt, thus
certain sections of the belt moving at different speeds to other sections, or large quantities
of belt being dumped between the idlers causing a loss of tension, or a combination of
these symptoms.
It has unfortunately become common practice to eliminate the symptom of
take-up movement by "fixing it" with a winch. This has the effect of making the design
perform as a rigid body, and it must therefore be over-designed to cater for the effects of
wave propagation. Failure to over-design, results in the shock wave destroying pulley or
drive components. Since this normally occurs in a progressive way (fatigue), it does not
always show itself as a dynamic problem.
Design of belt conveyor
63
benefiting from dynamic analysis
The advantages are generally seen in reduced costs and downtime. A
dynamic analysis can give the designer the confidence to reduce safety factors,
thereby lower the specifications for belting and allowing an increase in idlers spacing,
thus reducing power use and component spares holding. It should be noted that
conveyors set up with dynamic analysis techniques are operating with a safety factor
for the belt of less than five.
FIG. 4 A typical examples of theoretical and actual readings. The take-up
movement is an ideal test for model accuracy as it can be measured
in the field, giving substantial justification for confidence
in the modeling.
Design of belt conveyor
64
14.3 Conclusion
Setting up a basic model although very time consuming and highly complex,
allows the designer greater freedom in designing the system. A full analysis needs upward
of 100 runs to satisfy all the possible combinations of events. Today's computers require
two hours to complete a single analysis, and a full analysis would therefore take in the
region of 200 working hours.
When the basic analysis has been concluded, and problems like shock wave
propagation, excessive take up or negative belt tensions have been identified, it is then
possible to test various solutions. What if a brake were installed. Where is the best position
for a brake? Should we increase system inertia? All these questions can be investigated.
Once this exercise has been completed, it is then possible to show the results
of the full analysis using computer simulated models which, through a time base, can
display the operating parameters allowing for the correct formulation of the control
philosophy.
By using the simulation approach right from the design stage, the designer
can advance the roll of the conveyer to greater heights and lengths. Eliminating transfer
points and pushing the conveyor to higher speeds, knowing that he is not sacrificing
safety, will ensure that the final system will provide cost-effective bulk transportation for
many years to come.
Design of belt conveyor
65
15. Bibliography:
www.nbelts.com
www.conveyorkit.com
www.pro-belt.com
Material handling systems
Design data book